The present invention relates generally to paste systems and more specifically to a paste transfer system for transferring bonding paste or other material to an electronic die to be bonded to a substrate.
In bonding systems which bond a die to a substrate or a lead frame, the pattern for the bonding paste must be developed for each unique sized die. Generally the paste is applied as a plurality of dots resembling a shower head. To improve upon this process a star fish pattern has been used as described in U.S. Pat. No. 4,803,124 to KUNZ. Using either method, the pattern is specifically designed for the size of the die. This is not a problem on large runs, but for smaller runs through an automatic bonding machine, there is a lot of down time in developing the appropriate characteristics of the bonding paste pattern.
In prior systems, the bonding material is applied to the substrate or carrier surface generally prior to the bonding site as illustrated in U.S. Pat. No. 4,797,994 to Michaud et al. or at the bonding site as illustrated in U.S. Pat. No. 4,857,133 to Mullen. These systems apply the paste to a stationary substrate and thus increase the cycle time and reduce throughput.
The die paste may have a viscosity in the order of 20,000 to 100,000 CENTEPOISE. Typical examples of die paste are: silver glass composition, silver polyamide, gold substitute in glass, or glass paste alone. The handling and dispensing of these pastes is very difficult, as well as providing a uniformed layer without voids which trap air. A metering apparatus for dispensing silver glass paste is specifically described in U.S. Pat. No. 4,974,754 to Wirz.
Prior to the shower head application of paste on the substrate, paste was applied to the bottom of the die substrate by dipping a tool into a paste pot and then contacting the substrate. The paste pot generally included a disc which is rotated relative to a doctor blade to create a uniform layer of paste. Once the layer has been established, the disc is stationary and a die tool is dipped into the layer on the disc and removed to transfer a layer of paste to the die tool. The tool would then contact the substrate to transfer the paste to the substrate. The paste tool moved in tandem to the die pick up. The inability of these paste pots to form an appropriate layer of materials.
In addition to limitation that shower heads are designed for each specific size and shape of die, they also have the difficulty of forming patters on a small die sizes. Wherein multiple chips are provided in a single package or cavity, the shower head cannot provide these individual patterns much less in a common package. The alignment of the shower heads to the bond site offers another area for possible misalignment.
Thus an object of the present invention is to provide a paste transfer system which minimizes the set-up time.
Another object of the present invention is to provide a bonder paste system which increases through-put.
Still another object of the present invention is to provide a paste transfer system that can accommodate different size dies on a common substrate or package.
A further object of the present invention is to provide a paste transfer system wherein the bonding paste is applied to the die prior to being bonded to a substrate.
Still another object of the present invention is to provide a paste transfer system wherein the bonding paste is applied to the die on the fly.
An even further object of the present invention is to provide a paste transfer system designed for silver glass or other bonding adhesives which require shear for controlled transfer.
These and other objects of the invention are attained by a bonding system including a transport transporting a die from a supply location through a paste station in a controlled manner to a bonding site on the substrate. The paste station applies a first layer of bonding paste on a surface of the die as the die moves through the paste station. The control for the transport controls orientation which the die passes through the paste station and the orientation of the die at the bonding site. This allows the die to be oriented such that it passes through the paste station along as its shortest dimension while allowing reorientation before being bonded at the bonding site. A controller includes parameters for the die transport and the paste station as function of die size and implements the parameters as a function of the bonding sequence of dies. This allows changing of the pasting parameters on the fly for various size and a sequence of various size dies.
The paste station has a layer of bonding paste through which the die passes to form the first layer on the die. The height of the die is varied as it passes through the second layer. The speed at which the die and the second layer of bonding paste move relative to each other is also adjusted to produce the desired thickness of the first layer on the die. Preferably, the second layer is provided by a roller rotating into and out of a reservoir.
Shear is produced in the second layer to affect transfer of the bonding paste from the reservoir to the die. The shear is produced by a difference in speed between the die and the bonding paste layer as well as the angle of entry and angle of exit of the die to the paste. The speed of the roller and the speed of the die are controlled to produce the desired shear. For larger dies, the die may enter and leave the paste more than once to produce individual plateaus spaced from the edge of the die. The producing of shears is specifically important wherein the bonding paste is silver glass bonding paste.
A method for applying a bonding paste to an electronic die to be bonded to a substrate includes a) providing a first layer of bonding paste, and b) moving a die and the first layer into contact at a first surface of the die and relative to each other to create a shear in the first layer sufficient to form a second layer of bonding paste on the first surface of the die.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.